A desirable solar cell geometry referred to as an interdigitated back contact (IBC) cell comprises a semiconductor wafer and alternating lines (interdigitated stripes) respectively coinciding with regions with p-type and n-type doping. This cell geometry has the advantage of eliminating shading losses altogether by putting both contacts on the rear side of the wafer that is not illuminated. Further, contacts are easier to interconnect with both contacts on the rear surface.
Another desirable solar cell architecture involves the use of silicon heterojunction or tunnel junction contacts. An example of such architectures is the HIT (heterojunction with intrinsic thin layer) cell structure. In a front emitter form of this structure, a silicon wafer is contacted on both sides by a thin intrinsic hydrogenated amorphous silicon (a-Si:H) layer, which serves as a surface passivating layer as well as a charge carrier transport layer. On the front of the cell, a semiconductor layer doped to the opposite doping polarity of the base substrate is applied, forming a heterojunction emitter. On the rear of the cell, a semiconductor layer doped to the same doping polarity as the base substrate is applied, forming a base contact. These layers can then be contacted with transparent or metallic conducting layers to extract current from the solar cell. In the tunnel junction cell, the intrinsic a-Si:H layer is replaced with a thin high bandgap material. In the case of the heterojunction cell, charge carrier transport occurs via both hopping conduction in traps and band conduction in the intrinsic a-Si:H layer, while in the case of the tunnel junction cell, charge carrier transport occurs via quantum mechanical tunneling. Despite this difference, the cell structures are somewhat similar and importantly can be manufactured in low temperature processes because they do not require dopant diffusion.
Heterojunction or tunnel junction solar cells cannot achieve outstanding efficiencies because they still require front side contacts. First, the presence of a contact on the front side reduces efficiency due to blocking or shading of the incoming light by the necessary metal grids which extract the generated current. Additionally, the presence of a front electrical contact requires that the front of the cell be simultaneously optimized for electrical, light absorption, and passivation properties, often producing a compromise that affects cell performance.
Presently, silicon solar cells with the highest efficiency are those based on combining an interdigitated all back contact structure with silicon heterojunction contacts. Silicon solar cells have been reported with efficiencies as high as 25.6%. While the processing of these high efficiency IBC solar cells were not discussed in any detail, the manufacturing costs are likely to be relatively high since the known processing techniques that could be applied in each case appear to be somewhat complicated with various masking and vacuum processing steps required.
Many solar cell structures, including the IBC structure, rely upon thin fingers of metal to collect current. In the case of the IBC cell, these thin fingers are interdigitated on the back of the cell, and often resulting from a single metal deposition that has been patterned to reveal the finger structure. Since the fingers exist on the same layer, the area of each finger can only be about ½ of the full area of the cell, but actually less than ½ because there is need for an insulation region (isolation gap) between the fingers. Combining this with the fact that current must travel along the narrow fingers, very high conductivity layers are required for the fingers. The solar industry resorts almost exclusively to silver, an expensive metal, to address the need for high conductivity, thus low resistance. Additionally, larger cell sizes exacerbate issues associated with resistance in the metal contacts of the solar cell.
There have been attempts to provide improved metallization for solar cells. A multilayer metallization applies each contact metal (base, emitter) as a contiguous sheet of metal over the entire cell with vias in the bottom contact metal sheet allowing the top sheet to make localized base contacts, such vias made with a photolithographic process. However the photolithographic process is expensive and not suited to solar cell manufacturing. Another method applies relatively thick metals in cost effective ways, but does not address approaches needed for make useful back contact cells.
Therefore, there is a need to provide multilevel metallization systems and methods for back contact solar cells that can be low cost. In U.S. patent application Ser. No. 15/068,900 filed on Mar. 14, 2016, laser processed back contacts were discussed. Further improvements to systems and methods for forming rear emitters for solar cells are discussed herein.